Modern horizontal axis wind turbines comprise a tower construction mounted on a ground foundation or on an offshore foundation structure. The tower construction carries a nacelle carrying a hub with a number of wind turbine blades, often three wind turbine blades. The hub is mounted rotatably on the nacelle about a substantially horizontal axis in such a manner that the hub rotates as a consequence of the wind turbine blades catching the wind. The nacelle houses various components used for converting this rotational movement into electrical energy, such as a generator and possibly a gear arrangement.
The nacelle is mounted on the tower construction in a rotatable manner, via a yawing system, in order to allow the wind turbine blades to be directed in accordance with the wind direction. The yawing system typically comprises a large toothed ring and one or more yaw drives, each yaw drive comprising an output gear wheel arranged in engagement with the large toothed ring. The large toothed ring may be arranged on the tower construction, while the yaw drive(s) is/are mounted on the nacelle. As an alternative, the large toothed ring may be arranged on the nacelle, while the yaw drive(s) is/are mounted on the tower construction. In any event, when the output gear wheel of the yaw drive(s) is/are rotated, the nacelle is rotated due to the engagement between the large toothed ring and the output gear wheel(s) of the yaw drive(s), i.e. yawing movements are performed.
When yawing movements are not performed by the yawing system, it is desirable to maintain the yawing system in the selected position. This may, e.g., be obtained by applying a preload to the yawing system, where the preload force must be overcome in order to move the nacelle relative to the tower construction. As an alternative it may, e.g., be obtained by means of a separate yaw braking system. In the case that a preload is applied, the size of the preload force must be selected and balanced in such a manner that, on the one hand, the preload force is sufficiently high to maintain the nacelle in position when it is not desired to perform yawing movements, and, on the other hand, the preload force is sufficiently low to allow yawing movements without having to transfer excessive torque between the output gear wheel(s) of the yaw drive(s) and the large toothed ring.
EP 1 571 334 A1 discloses a wind turbine yawing system and yawing process. The yawing system comprises a gear ring fixed to the tower, at least one geared motor meshed with the gear ring through a gear wheel, at least one active braking module, and at least one passive braking module. Each active braking module comprises at least one braking element shiftable between a first position and a second position, according to braking instructions, such that the active braking module exerts a first braking force when the braking element is in the first position, and a second braking force when the braking element is in the second position, the second braking force being greater than the first braking force. Each passive braking module exerts a constant braking force. Thus, the active braking module(s) allow(s) the total braking force to be adjusted. However, in the case that it is desired to perform yawing movements, it is still necessary to overcome the braking force provided by the passive braking module(s).
EP 1 662 138 A1 discloses a speed reducer for use in a yaw drive apparatus for a wind power generation apparatus. According to one embodiment, the speed reducer comprises a plurality of brake mechanisms attached to a nacelle housing. Each brake mechanism is constituted by a fluid cylinder connected to a switching valve through a supply/discharge passage.